1. Field of the Invention
The present invention relates to a spectrometer which is used to measure a spectrum of light supplied from a light source, in particular, a spectrometer having higher resolving power without enlarging a size of the apparatus.
2. Description of a Related Art
A spectrometer disclosed in Japanese patent application publication JP-A-11-132848 is known as a conventional optical apparatus which is used to measure a spectrum of light supplied from a light source. FIG. 31 shows a constitution of the spectrometer.
As shown in FIG. 31, spectrometer 200 has slit board 201, collimator lens 202, beam splitter 203, diffraction grating 204, mirror 205, magnifier lens 206, and line sensor 207.
In spectrometer 200, light supplied from a light source passes through slit board 201 and is changed into parallel light by collimator lens 202. The parallel light passes through beam splitter 203 and is incident into diffraction grating 204. The parallel light incident into diffraction grating 204 is diffracted toward beam splitter 203 as a first diffraction light (a single pass).
A part of the single pass is reflected by beam splitter 203 toward a direction shifted by a small angle from the optical axis of collimator lens 202. On the other hand, the rest of the single pass passes through beam splitter 203, collimator lens 202, mirror 205 and magnifier lens 206, and then focuses on a first range of channels of line sensor 207 to form a spectrum image.
The single pass reflected by beam splitter 203 is incident again into diffraction grating 204 and then diffracted toward beam splitter 203 as a second diffraction light (a double pass).
A part of the double pass is reflected by beam splitter 203 toward a direction shifted by a small angle from the optical axis of collimator lens 202. On the other hand, the rest of the double pass passes through beam splitter 203, collimator lens 202, mirror 205 and magnifier lens 206, and then focuses on a second range of channels of line sensor 207, which is different from the first range, to form a spectrum image.
Although the above-mentioned document disclose no concrete example of diffraction grating 204 for realizing a large diffraction angle as shown in FIG. 31, an Echelle grating is suitable for realizing such a large diffraction angle.
However, there are some problems in the case where the Echelle grating is used. That is, a number of groove lines or density of groove lines needs to be increased in order to make a diffraction grating which has a large diffraction angle and which can be used in a Littrow arrangement, however it is difficult to produce an Echelle grating having a large number of groove lines. On this account, diffraction light of a high diffraction order needs to be used in order to make the diffraction angle larger with a small number of groove lines. As a result, a difference between a wavelength of diffraction light of a certain diffraction order and a wavelength of diffraction light of the adjacent diffraction order, in other words, an FSR (free spectral range) becomes small.
Assuming that a wavelength of light incident into a diffraction grating is xcex1 and a diffraction order of the incident light is m1, and a wavelength of diffraction light is xcex2 and a diffraction order of the diffraction light is m2, every diffraction light is diffracted into the same direction when the following expression (1) is satisfied.
m1xc2x7xcex1=m2xc2x7xcex2xe2x80x83xe2x80x83(1)
When expression (1) is satisfied, a wavelength difference xcex94xcex between a wavelength xcex1 and a wavelength xcex2, that is, an FSR is expressed by expression (2).
xcex94xcex=xcex1/(m1+1)xe2x80x83xe2x80x83(2)
By the way, if incident light is diffracted by using an Echelle grating and the diffraction order m1 is a very large number, for example, 100, every wavelength xcex2 satisfying m1xc2x7xcex1=m2xc2x7xcex2 (m2: an integer) is observed at the same position as that of wavelength xcex1. For this reason, when plural spectrum lines exist in a wavelength range near to the wavelength xcex1, discrimination of those spectrum lines becomes difficult.
Concretely, for example, when light from an iron (Fe) hollow-cathode lamp is incident into an Echelle grating having a groove line number of 85.34 lines/mm, the diffraction angle of the 93rd order diffraction light having a wavelength xcex1=248.3271 nm of this lamp is different only by 0.018 degrees from the diffraction angle of the 55th order diffraction light having a wavelength xcexc2=419.9098 nm as shown in FIG. 32.
Thus, in a spectrometer in which the above-mentioned FSR is small, a difference of diffraction angles of diffraction lights of adjacent two diffraction orders is very small. In the case where the light source has plural emitting spectral lines as in the iron (Fe) hollow-cathode lamp, spectrums of respective diffraction lights are overlapped to each other and those spectrums can not be separated. As a result, it is difficult to identify the correspondence relation between the spectrums.
For example, even if a real spectrum having different wavelengths is distributed as shown in FIG. 33(a), the spectrum observed by the spectrometer having a small FSR has wavelengths as shown in FIG. 33(b) because a difference between diffraction angles of diffraction lights of diffraction orders adjacent to each other is very small in this spectrometer.
Further, in the spectrometer having a small FSR, there is a problem that a baseline portion of each spectrum can not be measured precisely because plural spectrums of distant orders are observed close to each other. As shown in FIG. 34, spectrum S1 exists in the neighborhood of spectrum S2 to be measured and a shape of spectrum S1 overlaps with a shape of spectrum S2 to be measured.
Although there are the above-mentioned problems when light having plural emission spectrum lines is incident into an Echelle grating, when light having only one emission spectrum line is incident into an Echelle grating, an output of the light can be detected.
In order to solve the above-mentioned problems, there is proposed the constitution as shown in FIG. 35 including a preliminary dispersion element, which makes plural emission spectrum lines into one emission spectrum line, before the light is incident into spectrometer 1 as disclosed in JP-A-11-132848. The preliminary dispersion element corresponds to means for making one line, in which prism 4 is arranged between two lenses 2 and 3.
This preliminary dispersion element makes the light that passed through lens 3 to be dispersed by prism 4 and then pass the dispersed light through lens 2 so that one-lined light is incident into slit 1a of spectrometer 1.
However, one prism alone can not completely remove the overlapped spectrum as mentioned above because the resolving power is not enough, that is to say, the performance for dividing neighboring spectrums is not enough. Therefore, it is considered to use plural prisms for improvement of the resolving power. However, There is a problem that the prism constitution becomes larger.
Besides, since the light passing through the prism generates some thermal change (temperature change), a refractive index of the prism also changes along with the temperature change. Accordingly, the light that passed through the prism at a position once fixed may become not to pass through slit 1a. In other words, the spectrum may become not to be measured.
Alternatively, preliminary spectrometer 5 as shown in FIG. 36 without using a prism may be used as means for making a single line. The preliminary spectrometer 5 is designed such that the light that passed through slit 6 is reflected by concave lens M1 into diffraction grating 7, the light diffracted by diffraction grating 7 is reflected by concave lens M2 and further reflected by reflecting mirror 8, thus the single-lined light is incident into conventional spectrometer 1 through slit 9.
Although spectrometer 1 is provided with sufficient resolving power in this manner, intensity of the single-lined light is decreased, and therefore, measurement of the light intensity by a photometer of spectrometer 1 becomes difficult.
Since the spectrometer as disclosed in JP-A-11-132848 is designed such that the light that passes through the part reflection mirror is diffracted by the diffraction grating, there occurs a problem that intensity of the light having a spectrum to be measured becomes weak and a S/N ratio becomes small when the light passes through the part reflection mirror. For example, if the light is diffracted three times by the diffraction grating, intensity of the diffracted light is decreased to only 0.55% of intensity of the incident light.
Furthermore, in the spectrometer as disclosed in JP-A-11-132848, a spectrum line not diffracted by the diffraction grating, a spectrum line diffracted once and a spectrum line diffracted twice are observed close to each other. Further, intensity of the light of less diffraction number of times is stronger. As a result, there is a problem that a baseline portion of once diffracted spectrum S1 overlaps with a baseline portion of twice diffracted spectrum S2 as shown in FIG. 34, thus a baseline portion of the spectrum of the desired light can not be measured precisely.
The present invention has been accomplished under the above-mentioned circumstances, and an object of the present invention is to provide a spectrometer which can realize higher resolving power without enlarging a size of an apparatus.
In order to solve the above-mentioned subjects, a spectrometer according to the first view of the present invention comprises detection means having plural light detection elements arranged in at least one direction; first diffraction means for diffracting incident light with a first diffraction order and outputting the diffracted light to the detection means, grooves being formed in the first diffraction means with such density that the detection means can detect the specified spectrum line; and second diffraction means for diffracting incident light with a second diffraction order higher than the first diffraction order and outputting the diffracted light to the first diffraction means, grooves being formed in the second diffraction means with less density than that in the first diffraction means.
Further, a spectrometer according to the second view of the present invention comprises detection means having plural light detection elements arranged in at least one direction; a collimating means for changing light supplied from a light source into parallel light; first diffraction means for diffracting the parallel light including a specified wavelength component into a predetermined direction; second diffraction means for diffracting the parallel light output from the first diffraction means into the first diffraction means so that the parallel light goes and returns a predetermined times between the first diffraction means and the second diffraction means; and focusing means for focusing the parallel light, which has gone and returned the predetermined times between the first diffraction means and the second diffraction means, on the detection means.
In the spectrometer according to the present invention having the first and second diffraction means, the specified wavelength component included in the light supplied from the light source is diffracted a predetermined times by each of the first and second diffraction means to obtain a spectrum image of the specified wavelength component.
On the other hand, in a conventional spectrometer having only one diffraction means, the specified wavelength component included in the light supplied from the light source is diffracted a predetermined times by this diffraction means to obtain a spectrum image of the specified wavelength component.
Due to such a difference, according to the present invention, the specified wavelength component included in the light supplied from the light source is diffracted more times than the conventional spectrometer. Therefore, an angle dispersion value larger than that in the conventional spectrometer can be obtained. That is to say, a smaller dispersion value can be obtained without lengthening a focal length of the collimating means. Accordingly, higher resolving power can be realized without enlarging a size of the apparatus.
In particular, in the case where the first diffraction means includes a holographic grating which diffracts the parallel light incident from collimating means with an outgoing angle of 0xc2x0, a beam width of the parallel light is magnified when it is diffracted by the holographic grating. Therefore, still higher resolving power can be obtained along with a still larger angle dispersion value.